The purposes of conversion coatings on metal surfaces are to provide corrosion resistance and improve the adhesion of coatings. The conversion coating improves the adhesion of coating layers such as paints, inks, lacquers, and plastics. Traditionally, chromates have been utilized as conversion coating compounds. Chromates are known to exhibit acceptable performance on different types of metals or alloys, such as aluminum, steel, galvanized steel, and zinc-aluminum coated steel. Chromate conversion coatings are typically administered by contacting the metal surface with an aqueous solution containing hexavalent or trivalent chromium ions. Concerns have been raised regarding the pollution effect of chromate discharge into rivers or water ways by such processes. Due to the high solubility and the strongly oxidizing character of hexavalent chromium ions, conventional chromate conversion coating processes require extensive waste treatment procedures to control their discharge.
Attempts have been made to produce acceptable chromate-free conversion coatings. Chromate-free conversion coatings based upon complex fluoroacids and heavy metals are known in the art. However, they have not enjoyed wide spread commercial acceptance.
The use of non-chrome conversion coatings, based upon anionic polyacrylamide copolymers are known in the art. U.S. Pat. No. 5,412,011 discloses a nonchrome pretreatment for metallic surfaces which comprises anionic polyacrylamide copolymers, inorganic silicates and organo functional silanes. U.S. Pat. No. 5,518,555 discloses a metal pretreatment which is essentially free of silicate, chromium, fluoride and heavy metals which comprises an aqueous solution of anionic polyacrylamide.
While nonchrome conversion coatings which include anionic polyacrylamide provide effective pretreatment, they do exhibit limitations. The anionic polyacrylamides exhibit instability at extreme pH's. Also, anionic polyacrylamide pretreatments have been found to be incompatible with other water soluble polymers, such as polyvinyl alcohol which may be present in conversion coating treatments.